If a user leaves early, their track will be unfinalized and you will need to finalize it manually after the recording is finished. Once you have finished recording, you wait until all the other uploads are finished uploading then refresh the page. You will then see an unfinalized track from your guest leaving early. Click on it and it will finalize the track and make it show up in your cloud drive.
See our article on Recovering Unfinalized Tracks
Another option would be to have your user who is leaving early just mute themselves but keep the browser tab open. Then the tracks should all finalize properly the first time.